Instruments on the Chandra X-Ray Observatory

TODO: add example images.

There are two detectors on Chandra -
ACIS and HRC -
that can be used either with or without
one of the two grating arrays:
LETG or HETG.
The ACIS and HRC instruments are mounted on
the SIM,
which can be moved to place either of them at the
focal plane of the telescope (for some observations -
mostly for calibration but sometimes when extremely bright
sources are being observed - the instrument is moved
so that the instrument is not at the center of the
focal plane; the instruments can also be placed in
a safe position, shielded by the rest of the telescope,
when the particle background is high).
The gratings are mounted behind the Chandra mirrors,
and can be inserted into the light path when needed.

As a rule of thumb, the instruments are used for
the following science cases:

Observing many sources or a large source

The ACIS-I array is often used because of its size
and energy resolution. The HRC-I array covers a larger
area, but is less sensitive and has very-limited
spectral resolution.

High spectral resolution of bright sources

For those sources where high spectral resolution is required,
a grating is used. Although any grating can be used with
any instrument, the following two combinations are preferred,
since they provide the best performance:
HRC-I with the LETG or ACIS-S with the HETG.

High spectral resolution of faint sources

As using gratings reduces the efficiciency of the
telescope, they are not used with faint sources.
In this situation the common option is to use
ACIS-S with no grating.

High spatial resolution

The HRC-I is used when spatial resolution
is important.

There are specialized modes of operation
possible, such as CC
mode, which continuously reads out the ACIS chips to greatly
improve the time resolution but at the loss of 1 dimension
of imaging data, or using a sub-array to read out only a
subset of a chip.
These are not commonly used, as they
require specific conditions - normally a very-bright source -
and so are not described further here.

ACIS

The ACIS instrument
on the Chandra X-ray Observatory
contains 10
CCDs
arranged into two shapes (4 and 6 chips).
Although there is a great deal of flexibility in which chips
can used in an observation - due to telemetry limits
only a maximum of 6 chips could be used
at the start of the mission and this has now dropped to
5 due to thermal constraints
- observations with ACIS are split into two categories:
ACIS-I and ACIS-S.

Of the ten chips, eight of them
are front illuminated (FI), with the remaining two
- both in the ACIS-S array - being back illuminated (BI).
The general characteristics of the two different types of
chip are the same - that is pixel size (0.492 arcsecond
square) and number of pixels (1024 by 1024) - with the
main differences being:

energy resolution

At launch the BI chips had higher spectral resolution -
that is they were better at distinguishing between
X-ray photons with similar wavelengths.
This means that observing a source with a BI chip
is likely to lead to a more accurate model for
sources with a lot of structure - such as line
emission - in their spectra.
However, the difference
in resolution between the BI and FI chips has decreased
with time due to radiation damage during the early
part of the observation.

sensitivity

The BI chips are more sensitive at low energies (below about
1 keV), which means that they are often used for observing
"softer" sources: X-ray astronomers use the term "soft" to mean
sources with most of their emission at low energies (longer
wavelength) and
"hard" for sources with more emission at high energies (shorter
wavelengths),
in a similar manner to how optical astronomers use
the terms "red" and "blue".

background level

The background counts detected by the BI chips - which is a
combination of instrumental, particle, and cosmic components -
is higher than that of the FI chips in the same observation.

The standard mode of operating the ACIS chips is to read them out
every 3.4 seconds. The combination of source flux and instrument
sensitivity means that most pixels do not detect any counts in
each readout period, which means that when we detect a count
we can generally assume it represents a single photon.
An estimate of the energy of the incoming photon is found
by measuring the amount of electrons in the pixel (the "count").
If the source is bright enough then there is the possibility of
detecting two or more counts in a single readout; this is
referred to as "pile up" and results in strange behavior
(the source spectrum is artificially hardened - i.e. it contains
more high-energy counts than it should - and in the most
extreme cases images of the source become "cratered",
as the counts in the center of the source are lost).
One way to detect the presence of a bright source is to look
for "read out streaks": since there is no shutter on the
ACIS cameras, the detectors are still taking data during the
time it takes to read out a frame (this read out time is
about 0.04 seconds) which causes the whole column
to contain extra counts.

ACIS-I

The ACIS-I array contains four chips arranged into a square.
Since each chip is 8 arcminutes square, the ACIS-I array
covers a square 16 arcminutes on a side (the orientation
depends on the roll-angle of the telescope at the time
of the observation). All four chips are front illuminated (FI),
and are referred to as ACIS-I0 to ACIS-I3 or ACIS-0 to ACIS-3.
This set up is optimised for imaging experiments, and is
rarely used with a grating.

ACIS-S

The ACIS-S array contains 6 chips in a single row, so
it is 8 arcminutes wide by 36 arcminutes long. The full
array of chips is generally only used with the
HETG grating, but the central
chip (known as ACIS-S3 or ACIS-7) is often used
for imaging observations (i.e. with no grating), since
it is a back-illuminated (BI) chip.
The ACIS-S chips are referred to as ACIS-S0 to ACIS-S5
or ACIS-4 to ACIS-9.

HRC-I

HRC-S

The Chandra gratings

While the ACIS chips have some energy resolution - provided by
measuring the number of electrons in each "count" - the highest
spectral resolution is provided by the grating arrays. When used,
one of the two arrays is moved into the light path behind the mirrors
and the light is diffracted by the transmission gratings in the array
onto the detector (normally either ACIS-S or HRC-S).
Since the light has been diffracted, the location of the count on the
detector can be used to determine the energy - or wavelength - of the
incoming photon. The gratings provide an energy resolution of
approximately one part in a thousand.

LETG

The LETG spectrometer
contains regularly-spaced wires of gold and are designed for observing
sources in the 0.08 to 2 keV energy range, or 6 to 150 Angstroms.
The LETG is primarily used with the HRC-S.

HETG

The HETG spectrometer
contains two gratings, arranged at different angles so that the dispersed
spectra form an "X" pattern on the detector, which is normally ACIS-S.
The gratings are also made of gold but with much smaller spacings between
the wires, so that the energy range covered is 0.4 to 8 keV, or
to 1.5 to 30 Angstroms.

Further information

The 'What is Chandra doing now?' web site is developed by
the Science Data Systems group of the
Chandra X-ray Center.
The information on this site is intended for
educational purposes and should not be relied on for
scientific use.